Enhanced ultraviolet-B (UV-B) radiation may have multiple effects on both plants and animals and affect plant-herbivore interactions directly and indirectly by inducing changes in host plant quality. In this study, we examined combined effects of UV-B and herbivory on the defence of the mountain birch (Betula pubescens ssp. czerepanovii) and also the effects of enhanced UV-B radiation on a geometrid with an outbreak cycle: the autumnal moth (Epirrita autumnata). We established an experiment mimicking ozone depletion of 30% (a relevant level when simulating ozone depletion above Northern Lapland). Both arctic species responded only slightly to the enhanced level of UV-B radiation, which may indicate that these species are already adapted to a broader range of UV-B radiation. UV-B exposure slightly induced the accumulation of myricetin glycosides but had no significant effect on the contents of quercetin or kaempferol derivatives. Mountain birch seedlings responded more efficiently to herbivory wounding than to enhanced UV-B exposure. Herbivory induced the activities of foliar oxidases that had earlier been shown to impair both feeding and growth of moth larvae. In contrast, the contents of foliar phenolics did not show the same response in different clones, except for a decrease in the contents of tannin precursors. The induction of foliar phenoloxidase activities is a specific defence response of mountain birches against insect herbivory. To conclude, our results do not support the hypothesis that the outbreak cycle of the autumnal moth can be explained by the cycles of solar activity and UV-B.